Scanning method



SePL 15, 1959 s. A. scHERBATsKoY 2,904,629

SCANNING METHOD Filed April 28, 1952 [vom United States Patent() SCANNING METHOD Serge A. Scherbatskoy, Tulsa, Okla. Application April 28, 1952, Serial No. 284,677

2 Claims. (Cl. 1786.8)

The present invention relates to television transmitting and receiving systems and more particularly to a system for reducing the horizontal line structure in the image which is reproduced at the television receiver.

Under present television standards the image to be transmitted is scanned in a series of fields of parallel lines and the image is reproduced by means of a cathode ray tube at the television receiver by deiiecting the electrode beam yof the cathode ray tube in exact synchronism with the scanning means at the transmitter so as to set up a corresponding series of elds of parallel lines. With this arrangement both the image at the transmitter and the cathode ray tube screen at the receiver are continuously scanned along the same horizontal lines so as to produce pronounced bright lines in the television picture. The field of lines so produced is called a raster and is sometimes referred to as the horizontal line structure of the reproduced image.v The present television standards call for a eld of 525 horizontal lines and with a normal spot size for small cathode ray tubes the horizontal line structure is not too objectionable since the horizontal lines are almost contiguous. However, as the size of the television screen is increased, the distance between the horizontal scanning lines also increases and in large screen television receivers the horizontal line structure is very definite and pronounced and is very objectionable if the viewer is close to the screen. The present trend is to large screen television. In fact, the smallest cathode ray tube of one manufacturers line of television receivers has a diameter of 17 inches. Accordingly, it would be very vdesirable to provide an arrangement wherein the bright horizontal scanning lines, i.e., the horizontal line structure in the reproduced image, is substantially reduced, and it is a primary object of this invention to provide an arrangement of this type.

It -is another object of the present invention to provide a system for reducing horizontal line structure in television receiver images wherein the scanning fields are displaced in one direction at either a periodic or random rate to prevent continual scanning of the image area along the same horizontal lines.

It is a further object of the present invention to provide a new and improved system for reducing horizontal line structure in television receiver images whereby the contrast of the picture is increased and the appearance of the picture is greatly enhanced.

Brieiiy, in accordance with oneaspect of the present invention, there is provided at the television transmitting station a video signal generator having an image area to which is presented an image to be-transrnitted together with scanning means for scanning the image area in a series of fields of parallel lines. Connected in series with the conventional vertical scanning generator there is provided a vertical displacement means controlled either by a random frequency signal generator or a signal generator of predetermined frequency for displacing the scanning fields in a yvertical direction to f l ce 2,904,629

prevent continual scanning of the image area along the same lines. The composite television signal thus rproduced is transmitted by conventional means to the television receivers within the telecasting area and the television receiver is provided with image reproducting means, such as a cathode ray tube, and means operated in synchronism with the synchronizing signal components of the transmitted composite television signal for deflecting the electron beam of the cathode ray tube in two directions perpendicular to each other to reconstruct the transmitted image. In the receiver there is also provided means controlled in synchronism with the eld displacement means of the transmitter for varying the electron beam in the vertical direction in direct proportion to the scanning eld displacement. With this arrangement a small up-and-down displacement of the scanning pattern is produced so that successive images are reproduced by scanning patterns that are not identical. The horizontal line structure, which is visible on the screen of the conventional receiver, then disappears because the successive sweeps of the electron beam are not in exactly the same horizontal line position. Also, because the same shift is introduced in synchronism at the transmitter and receiver, no distortion of the image ocv curs.

The invention, both as to its organization and lmethod of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which: The single iigure of the drawing discloses in block diagram form a television transmitting and receiving system embodying the principles of the present invention.

Referring now more particularly to the drawing, the transmitter portion of the system of the present invention is shown in the left-hand half of the drawing, and a typical receiving station is shown in the right-hand half of this figure. Referring rst to the transmitter portion of the system, there is provided a suitable video frequency signal generator 10, which may be of any well known design and construction. For example, in the illustrated embodiment the generator 10 is of the low velocity image storing type commonly known as an orthicon type of video signal generator. The generator 10 comprises a suitable electron gun arrangement for producing an electron beam and focusing the beam of low velocity electrons on the target electrode 11 which is of the mosaic or cellular type and is enclosed within the evacuated envelope of the generator 10. A load resistor 12 is connected between the target 11 and ground and video frequency signals are developed across the load resistor 12 in response to the scanning of the target by the electron beam under the control of the beam defiecting elements. More specifically, the electron beam issuing from the gun structure 13 is deiiected` in the vertical direction by means of the Vertical dellecting coils 14a and 14h and the beam is deilected in the horizontal direction by means of the horizontal deliecting coils 15a and 15b, all of these coils being positioned about the neck of the cathode ray tube in a conventional manner so as to produce efficient deflection of the electron beam.

An optical system indicated generally at 18 is employed to focus the object indicated as the arrow 19 upon the surface of the target 11. As the electron beam sweeps across the target 11, the storage cells of the mosaic-like target 11 are successively discharged to produce currents through the load resistor 12 in proportion to the amount of charge acquired by each cell and hence in accordance with the light intensity of this particular area of the image.

f" across the resistor 12 are coupled to a video amplier Patented Sept. 15, 1959 The video signals thus produced` 20, which may be of any suitable design, and is adapted to. amplify the video frequency components. of the tele. vision signal Without distortion, and the output from the video amplifier is coupled to a miXer amplifier 21. A

synchronizing signal generator. 22 is provided to estab-V lish theconventional'horizontal andvertical synchronizingsignalsV and these synchronizing signals are also con`q nectedto themixer amplifier 21 so as to produce in the output. of this amplifier a composite television signaly consisting of the synchronizing signals which are superimposed upon the video signal during the synchronizing intervals of thesignal.

The. composite television signal which -is produced in the` output circuit of the mixer amplifier 21 is coupled to a transmitter 23 wherein the composite television signali is modulated upon a suitable high frequency carrier, either, by amplitude, phase, or frequency modulationand the modulated carrier wave iscoupled to the transmitter antenna system 24 for broadcast to the service area of the television transmitter station.

In order toV produce scanning currents in the deflection coils 15in, leb, 15a, and 15b, there is provided a, horizontal scanning generator 25 and a vertical scanning generator 216` which are operated in synchronis'm with. the. synchronizing signal generator 22. More specically, the4 horizontal signal generator 25 is controlled b y the horizontal synchronizing signals developed by the generator 22 and produces a saw-tooth scanning Wave which is impressed upon the horizontal deflection coils 1511i and'lb so as to deflect the electron beam in the horizontal direction. In a similar manner the vertical scanning generator 26 is controlled by the vertical synchronizing components of the synchronizingY signal developed by the generator 22 and produces a saw-tooth Wave at the vertical scanning rate which is impressed upon the vertical deilection coils 14a and 1 4b.

The modulated carrier wave broadcast by the antenna system 2,4 is picked up by the receiver antenna system 3.0 and supplied to an RF. amplifier and converter stage 31 in a television receiver located at a remote receiving point. The converter 31 operates to convert the received radio frequency signals into corresponding intermediate frequency signals which are amplified in the amplifier 32 and coupledk to the second detector 33. In the second detector 33A the composite television signalV is detected and the synchronizing signal components are supplied to a synchronizing signal separator 34 wherein the viert-ifV cal and horizontal synchronizing signals are separated and supplied respectively to the horizontal detlectionrcircuit 35 and the vertical deflection circuit 36. The composite videosignal is also supplied from the second de tector 33 to the video amplifier 37 wherein these signals arel ampliedtto a suitable level and supplied to the con; troligrid of a cathode ray tube indicated generally at The'cathode ray tube 38 is provided with a suitable beam? forming gun structure so as to produce an electron beam whieh strikes the end wall 39 of the cathode ray tube 38 and fiuoresces so as to reproduce the familiar tele.- vision` image. The horizontal deflection circuit 35. pro duces a saw-tooth scanning wave at the horizontal freqiiency whichv is supplied to the horizontal deiiection coils. itin and 4Gbj and thc Vertical dciccticn circuit 3 6u developsV a saw-tooth Wave at the vertioalscanning fre-A queney which isV supplied to the vertical deflection coils 41a and 41b. Accordingly, the electron beam of the cathode ray tube 38 is deflected intwo directions perpendieularA to each. other toV scan a rectilinear field and thereby reconstruct the transmitted image on the screen 39.

All of the above enumerated components may be of conventional well known construction, andaccordingly adetaileddescription thereof is considered urinecessaryV herein. lt Wili also be understood that 4any.conventional nicciccticn ci any ci inc abcvc-cnnincratcd, ccinpcncnin 4 may be made as desired without inuencing in any way the operation ofthe system of the present invention.

In order to reduce the horizontal line structure of the reproduced image produced on the end wall 39 of the receiver cathode ray tube 38, there is provided at the transmitter station a vertical displacement generator 50. The generator 50 may comprise either a free running oscillator or a driven oscillator operating at a certain predetermined frequency as described in more detail hereinafter, and the output of the vertical displacement generator is connected in series with the output circuit of the vertical scanning generator 26 so that the sum of the outputs of the two generators 26` and 50 comprises the total verticai defieetionv of the electron beam of the video signal generator 10. The output circuits of theV generators 26 and 50 may be combined in any suitable manner. For example, the vertical scanning generator 26n may be provided with an output transformer having an ungrounded secondary which is connected in series with the secondary of the outputA transformer in the verticaldisplacement generator 5t), the combined voltages produced' across Vthese secondary windings being impressed upon the deflection coils 14a iand 14.17., The magnitude ofthe output currentl developed by the vertical displacement generator 50 is relatively small as compared to the output of the vertical scanning generator 26 so that the scanning iield ofthe generatorV l0 is subject to a smailup and vdown displacement at a frequency determined by the frequency of vthevertical displacement generator 50. The vertical displacement generator 50 may either be operated atv a relatively low frequency as compared to the vertical sc anning rate or atf'a relatively high frequency as comparedto the horizontal deflection rate, and

these frequencies may either be synchronous or non: synchronouswith the corresponding scanning frequencies, The lWave shape of the output signal developed by the vertical displacement generator 50`may comprise either a` sinusoidal wave, a saw-tooth Wave, or a square wave. Also, the` vertical displacement generator 5G may be operated as a driven oscillator by lcorniiecting the output of the synchronizing signal generator 22 to the input of the vertical displacement generator 50, `Inithis manner the vertical synchronizing signals, whihV conventionaiiy recur` at the rate ofY 60 cycles per second, are employed to synchronize tbc verticalV disnlaccmcnt scncratcr which may bc crcrntcd ci c frcqncncv cf from 5 fc 35 cycles pcf Second Prcfcrablyz thc frcqucncy-Qi thc vertical displaccmcnt gcncrntnn is Sniicicntlv hielt tc prevent fiickcring cithc rcnrcdnccdinicsc While being Sniicicntly lcwic. ncrnnitc Sniff in tlicscanning nel@ during thc "criiccl .Icirncc intervals InA tnsccnnccticn it will be understood that if a square Wave type of verticaldis- Placcrncnfirrcvidcsi bv the gcncratcr 50 the abrupt chnecWhichfcc-cnrs atthclcaiine and trailing edges. cf. thc innate 'wave is, prctctablv Svnchrcnizen so. fc fenr iufngihc Vcrti'ca, retrace intervals o f thc signals dcvclcpcdbv thc synchronizing, generator 2;?-1nnd nnncr these conditions the verticei displac ment generator 5 0 Willbe Syncnrcnizcflfrcni thc ecnc. r 2.2.e

Thc Vertical. nisplaccnicnt generator 50. may also beV cncrniicd at n .Strictly random iatc.. Thus, accordati,ce` With cnc cnibcdirncnt ofthe invention arnndcni frcquency source 51, which nieyoniprise any-suitable noise. gcncrafcr cr cthcrdcvicc. having arancioni, frequency out- Pui is ccnplcsiicthc anirliiicr 5,2. vW11.cr ..iri the random'.

' frcqlicncv or trigger Inte.` of the verticall displa'ccnlcnt scncratc- In this' conncction it will .be understood thatY thc vertical dinnlnccrncnt gencratcr may comprise any. Suitnbc fyPc 0f cncillctcf which may 12e-triggered fron;

t inc. rendent iicnncncy. signal.'` passed byy the. low pass licr 5.- .In this,..c.cnnccticnit"wi1l bc understood that strictiyrnnint @initial-tica ianct 'accelerer so long as the period between exact repetitions of the vertical displacement signal is relatively long as compared to the corresponding scanning rate.

In order to produce a corresponding vertical displacement of the reproduced image at the television receiver station, there is provided in the television receiver a vertical displacement generator 55. The vertical displacement generator 55 may be similar to the vertical displacement generator 50 at the television transmitter and is provided with an output circuit which is connected in series with the output circuit of the vertical deflection circuit 36. Accordingly, the output of the vertical displacement generator 55 is added to the output of the vertical scanning wave provided by the vertical dellection circuit 36 so that the sum of these two waves is impressed upon the vertical deflection coils 41a and 41h and deects the electron beam accordingly.

The vertical displacement generator 50 at the transmitter and the vertical displacement generator 55 at the receiver may be synchronized by any suitable means. For example, in the illustrated embodiment a separate displacement signal transmitter 60 is provided to which is supplied a portion of the output wave of the vertical displacement generator 50. This vertical displacement signal is modulated upon a suitable radio frequency carrier and is transmitted from the transmitter antenna system 61 to the receiver antenna system 62, which latter element is coupled to a displacement signal receiver 63 at the receiver station. The receiver 63 is pro vided with a suitable radio frequency amplier channel and a suitable detector stage so that the vertical displacement signal is repro-duced in the output circuit of the receiver 63 and is impressed upon the vertical displacement generator 55 to control the frequency of operation thereof. In this connection it will be understood that other suitable means of synchronizing the vertical displacement generators 50 and 55 may be employed. For example, the output of the vertical displacement generator 50 may be modulated upon the main carrier wave developed by the television signal transmitter 23 in either a sub-multiplex arrangement or other suitable double modulation system. In the alternative, a direct Wire synchronization between the vertical displacement generators 50 and 55 may be eiected in situations where the transmitter and receiver are relatively close together. Also, a common alternating current power supply which interconnects the transmitter and receiver may be employed to effect synchronization of the generators 50 and 55 in a manner Well known to those skilled in the art.

From the foregoing description it will be understood that the present invention provides a relatively simple and efiiciellt system for substantially eliminating the line structure now present in television receiver images. By introducing a small vertical displacement of the scanning eld at the transmitter so that successive images are reproduced by scanning patterns that are not identical, and by providing a corresponding shift in the reproduced image at the receiver, horizontal lines in the reproduced image are not readily apparent and no distortion of the image occurs.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a television system wherein an image is scanned to form a video signal which is transmitted to a receiving point and there reconstructed into a visual image, the method of image scanning which comprises the steps of developing at the transmitter a main scanning signal of predetermined frequency and substantially sawtooth coniguration when plotted as a function of time, generating a noise signal characterized by random frequency components, adding the main scanning signal and the noise signal together to produce a resultant signal, using the resultant signal to eiect image scanning at the transmitter, transmitting to the receiving point suitable synchronizing pulses to permit reproduction at the receiving point of a counterpart of the main scanning signal, transmitting to the receiving point said noise signal, adding the received noise signal to the reproduced counterpart of said main scanning signal at the receiving point to produce a reconstructed resultant signal, and using the resultant signal to eiect scanning in the reproduction of the transmitted image.

2. The method of claim 1 which comprises the step of adjusting the ratio of amplitudes of the main signal and the noise signal to produce maximum scanningbeam displacement due to the noise signal approximately equal to the normal displacement between successive scans produced by the main scanning signal.

References Cited in the tile of this patent UNITED STATES PATENTS 2,112,684 Von Ardenne Mar. 29, 1938 2,222,934 Blumlein Nov. 26, 1940 2,251,525 Rosenthal Aug. 5, 1941 2,402,067 Mathes June 11, 1946 2,472,774 Mayle June 7, 1949 2,570,188 Aram Oct. 9, 1951 2,681,383 Loe H .v June 15,` 19,54 

